In recent years, a polarizing filter that is attached to the front side of the lens of a digital camera that is used for testing equipment, measuring equipment, medical equipment, or the like, and required to capture a very clear image, has been desired to function as an ultraviolet cut filter, a near-infrared cut filter, or an ultraviolet/near-infrared cut filter.
A technique that attaches a linear polarizing filter or a circular polarizing filter to the front side of the lens of a digital camera in combination with an ultraviolet cut filter, a near-infrared cut filter, or an ultraviolet/near-infrared cut filter has been normally used to deal with such a demand.
More specifically, a polarizing filter and an ultraviolet cut filter are attached to the front side of the lens of a digital camera when it is desired to obtain a filter that functions as a polarizing filter and an ultraviolet cut filter that can reflect (or absorb) ultraviolet rays.
A polarizing filter and a near-infrared cut filter are attached to the front side of the lens of a digital camera when it is desired to obtain a filter that functions as a polarizing filter and a near-infrared cut filter that can reflect near-infrared rays.
A polarizing filter, an ultraviolet cut filter, and a near-infrared cut filter (or a polarizing filter and an ultraviolet/near-infrared cut filter) are attached to the front side of the lens of a digital camera when it is desired to obtain a filter that functions as a polarizing filter and an ultraviolet/near-infrared cut filter.
However, since the total thickness of the filters considerably increases when a plurality of filters that differ in function are merely attached to the front side of a digital camera (e.g., digital single-lens reflex camera) in order to obtain a filter that functions as a polarizing filter and an ultraviolet cut filter, a filter that functions as a polarizing filter and a near-infrared cut filter, or a filter that functions as a polarizing filter and an ultraviolet/near-infrared cut filter, a vignetting phenomenon in which the captured image is adversely affected by the frame of the filter may occur. The vignetting phenomenon is particularly undesirable in the medical fields and the like for which a very clear and accurate image is required.
Therefore, development of a multifunctional polarizing filter that has a thickness almost equal to (desirably smaller than) that of a linear polarizing filter and a circular polarizing filter (hereinafter collectively referred to as “polarizing filter”) has been strongly desired.